1. Field of the Invention
This invention relates to oleaginous compositions and to the method of using said compositions to inhibit mildew and prevent the corrosion of metal. More specifically, this invention relates to oleaginous compositions comprising lubricating oils, organic solvents, corrosion inhibitors, rust-preventive agents, antioxidants, metal deactivators, mildew-inhibiting compounds and water-displacing agents. The oleaginous mildew and corrosion-inhibiting compositions of this invention are useful as coatings on various metal substrates including ferrous metals, aluminum, magnesium, and various other aluminums and ferrous alloys and particularly as mildew-inhibiting and corrosion-resistant coatings for aircraft and automotive frames. For example, as aircraft age, corrosion and mildew occurs in the internal structures which are not easily inspected or treated. Especially in harsh environments where humidity, salt and heat conspire to reduce metal parts to piles of oxide, fogging CPC's (Corrosion Preventive Compounds) that contain mildew-inhibiting agents into the internal spaces of airframes has been found to be effective in combating mildew and metal degradation. An objective of this invention was to develop an effective corrosion preventive compound (CPC) with mildew inhibition capabilities to prevent corrosion and simultaneously inhibit mildew growth on interior aircraft surfaces. The compositions of this invention reduce the cost of aircraft maintenance, extend aircraft's life cycle, reduce aircraft's down time and provide a healthy environment for the aircraft's crew. In comparison, current CPC's must be annually reapplied several times using time-consuming procedures. As an alternative to the current CPC's, this invention provides high performance, long lasting Corrosion Preventive Compounds (CPC's) for internal airframe applications to minimize the costs attributed to aging aircraft.
2. Background
Generally, CPC's comprise a barrier film containing corrosion inhibitors, various other additives and sometimes carrier solvents. Currently, film-formers for CPC's include natural and synthetic oils, oxidized petroleum fractions and polymers, depending on the desired application and performance requirements. Mineral oil as well as wool wax have proved useful, but more recent developments involve the use of polymeric resins, including the acrylics, silicones, urethanes and other proprietary materials. Most of these film formers provide a physical barrier to the corrosive environment, but cannot prevent the slow diffusion of corrosive agents through the film. Some films, particularly films containing the naturally derived materials, are not resistant to oxidation and require antioxidant additives to protect the film from degradation. In fact, without these and other additives, the barrier films often provide poor corrosion-resistance. The blending of additives in the film is usually the answer to superior performance with minor differences in structure often producing major effects in staving-off the corrosive attack of the environment.
In addition, corrosion preventive additives include not only anodic and cathodic inhibitors, but also acid acceptors and chelating agents. These materials provide a synergism with the film former that often produces outstanding corrosion protection. For example, calcium and barium salts of sulfonic acids (such as the alkylbenzenesulfonates and dinonylnaphthalene sulfonates) are outstanding metal deactivators resulting from the strong adsorption of the sulfonate group. The non-polar portion of the molecule tends to shield the metal surface from ionic attack of various environmental species. Phosphate compounds have been used, most recently, as a difunctional additive where the distance between the phosphate moieties was optimized for a particular resin system. In addition, vapor phase corrosion inhibitors (such as the dicyclohexylammonium compounds, various amines, and benzoates) can be useful in CPC films especially for internal applications where near-stagnant atmospheres exist.
Many of these CPC's contain carrier solvents which require evaporation to deposit the protective film. However, the use of solvents is regulated in many locations either by content (e.g. grams per liter volatile organic compounds (VOC)) or by vapor pressure. In addition to the solvent limitations, some additives previously used for their exceptional performance (such as barium sulfonates) are cited because of their heavy metal content. Substitute vehicles (such as water-borne resins) and substitute additives (such as calcium sulfonates) are possible, but only when the critical properties of the CPC performance are well understood.
Moreover, the formulation of CPC's has limitations. Higher concentrations of many additives results in higher viscosities causing the products to suffer performance problems. Ineffective water-displacement, incomplete crevice penetration, and poor sprayability are some of the problems that sooner or later contribute to the CPC's failure. Another approach to applying more corrosion-preventing additives is to use products that dry to thicker films, however, thicker CPC's attract hygroscopic dust and dirt that adds considerable weight to small aircraft which leads to maintenance problems such as the inability to inspect a surface. Further, there are several failure mechanisms for CPC's. Hard films fail when thermal expansion, mechanical movement or fatigue causes cracking of the metal substrate. Soft films fail when water slowly permeates and dissolves or emulsifies the CPC. Slow diffusion of environmental corrodents through a film will sooner or later initiate corrosion, damaging the film and allowing more direct attack on the surrounding metal. Some films flow sufficiently to heal themselves in spite of repeated physical film damage, however, this also means that flow occurs when there is no damage, resulting in decreasing film thickness and subsequent loss of the corrosion preventive properties. Further, some additives to the CPC's catalyze the hydrolysis of film formers which leads to porosity or even complete destruction of the film. Even atmospheric oxidation of the film or UV radiation induced failure can occur prior to the expected life of the film.
In addition to corrosion, there is the serious problem of mildew. Mold and mildew are simple microscopic organisms that grow anywhere if they have adequate moisture, nutrients and appropriate temperature. Many spores of mold and mildew are present at all times in indoor and outdoor air. These spores can settle, germinate, and grow wherever good growth conditions are found. It is reported that fungi appear to be able to use certain operational fluids, such as hydraulic fluid (MIL-PRF-83282) and corrosion preventive compounds (CPCs) as nutrients; see B. Little, R. Ray and J. Lee, “An Overview of Microbiologically Influenced Corrosion in Aircraft”, Naval Research Laboratory, Stennis Space Center, Technical Report No. A709314; (1997). Mildew growth is a serious problem particularly in hot, wet, and humid environments. It causes chronic health problems for the aircraft crew, requires excessive maintenance efforts, damages protective paint coatings, and damages bare metal surfaces. One study of fungal contamination on the interior surface of H-46 and H-53 rotary-wing aircraft at the Naval Air Depot, Cherry Point, N.C., isolated eight genera of micro fungi; see Lavoie, M. D., Little, B. J., “Fungal Contamination of H-53 Aircraft”, Naval Research Laboratory, Stennis Space Center, Technical Report NRL/MR/7333-96-7725, (1996). The study also indicated that some corrosion on unprotected aluminum surfaces could be attributed to bacterial and fungal growth. One of the isolated fungi (Aureobasidium) from the H-53 is known to cause superficial discoloration on latex paint; see R. A. Zabel, and F. Terracina, “The Role of Aureobasidium Pullulans in the disfigurement of latex paints”, Development in Industrial Microbiology, Vol 21, Pages 179-190, (1980).
Corrosion preventive compounds (CPCs) are used in various applications for corrosion control and prevention. A modified corrosion-preventive composition with mildew inhibitor capabilities has been developed to prevent corrosion and inhibit mildew growth on the interior of metal surfaces. Combining mildew inhibitors and corrosion-preventive compounds in one composition advances the efforts for controlling two major problems particularly in aircraft maintenance (mildew growth and corrosion).